Make numeric form of PG version number readily available in Makefiles.

Expose PG_VERSION_NUM (e.g., "90600") as a Make variable; but for
consistency with the other Make variables holding similar info,
call the variable just VERSION_NUM not PG_VERSION_NUM.

There was some discussion of making this value available as a pg_config
value as well.  However, that would entail substantially more work than
this two-line patch.  Given that there was not exactly universal consensus
that we need this at all, let's just do a minimal amount of work for now.

Back-patch of commit a5d489ccb7, so that this
variable is actually useful for its intended purpose sometime before 2020.

Michael Paquier, reviewed by Pavel Stehule

configure

@@ -627,6 +627,7 @@ ac_includes_default="\
 
 ac_subst_vars='LTLIBOBJS
 vpath_build
+PG_VERSION_NUM
 PROVE
 OSX
 XSLTPROC
@@ -15549,6 +15550,7 @@ _ACEOF
 
 
 
+
 # Begin output steps
 
 { $as_echo "$as_me:${as_lineno-$LINENO}: using compiler=$cc_string" >&5

configure.in

@@ -2128,6 +2128,7 @@ AC_DEFINE_UNQUOTED(PG_VERSION_STR,
 tr '.' '	' |
 $AWK '{printf "%d%02d%02d", $1, $2, (NF >= 3) ? $3 : 0}'`"]
 AC_DEFINE_UNQUOTED(PG_VERSION_NUM, $PG_VERSION_NUM, [PostgreSQL version as a number])
+AC_SUBST(PG_VERSION_NUM)
 
 
 # Begin output steps

src/Makefile.global.in

@@ -38,6 +38,7 @@ all:
 # PostgreSQL version number
 VERSION = @PACKAGE_VERSION@
 MAJORVERSION = @PG_MAJORVERSION@
+VERSION_NUM = @PG_VERSION_NUM@
 
 # Support for VPATH builds
 # (PGXS VPATH support is handled separately in pgxs.mk)